The present invention relates to loading assemblies used for applying selected loads to test specimens. More particularly, the present invention relates to a loading assembly capable of applying high loads, yet compliant for disturbing input motions.
A loading assembly generally includes an actuator and connecting elements to connect the actuator to the test specimen in order to apply a selected force or moment thereto. When hydraulic actuators are used, substantial loads can be applied. For instance, loading assemblies are generally known and used for simulating loads experienced by cars, trucks, motorcycles, planes, etc, or parts therefor. In the case of cars, trucks and motorcycles, the loading assembly can be used to simulate loads applied to the vehicle as the vehicle traverses a road or other course.
Although the loading assemblies mentioned above can apply high loads to simulate forces and moments applied to a test specimen such as a motor vehicle, there exists other loads which as of yet have not been properly simulated. For instance, racecars include air foils that are designed to generate substantial downwardly directed forces in order to help the racecar maintain contact with the road. However, in the presence of this downwardly directed force, the racecar may experience rapid chassis disturbances for example from aerodynamic blanking from another car in front, changes in the racecar pitch due to braking and centrifugal accelerations due to cornering at the friction limits of the tires. Although hydraulic actuators are well suited for applying a load that simulates the magnitude of the downforce exerted upon the racecar, the actuator is too stiff in the presence of these displacement disturbances. In other words, for higher frequency (e.g. greater than 2 Hertz) input velocity disturbances or displacements of the vehicle, the actuator and connecting components generally act as a rigid connection.
Although it has been known to mechanically soften an actuator by using a coiled spring connected in series with the actuator and a test specimen, or using a bending beam assembly, these techniques are generally unsatisfactory. In many instances, a long stroke actuator is needed in order to compress the coiled spring to generate the desired spring rate. This limits the effective operating range of the loading assembly. In addition, significant changes in the spring rate may require replacement of the mechanical spring. The spring must also be fully deflected in order to apply full load.
Accordingly, there is a need for an improved loading assembly that can apply substantial loads to a test specimen, yet be compliant for high frequency input disturbances.
A loading assembly for applying a load to a test specimen includes a container having a volume of compressible fluid for providing a spring. A cylinder having a bore operably connected to the container. A piston is slidable in the bore and is operably coupleable to the test specimen. A flow control valve fluidly coupled to the bore selectively provides fluid to the bore to displace the piston. A controller controls the valve to cause non-resonant displacement of the piston.
In one embodiment, the container comprises an accumulator having a volume of compressible gas. In another embodiment, the volume of compressible liquid in the container is equal to or greater than a maximum volume of a chamber defined by displacement of the piston in the bore.